Conventionally, an isolated switch-mode power supply device converts an inputted voltage into a desired voltage by switching a switching element and outputs the desired voltage. Such an isolated switch-mode power supply device employs a technique of burst-controlling the switching element in a standby mode in order to reduce power consumption in the standby mode. According to this technique, in the standby mode, an oscillation period in which switching of the switching element is performed at a predetermined cycle and a switching pause period in which the switching of the switching element is temporarily stopped are repeated. As this reduces the number of switching per unit time, it is possible to reduce a switching loss per unit time, and as a result, power consumption in the standby mode can be reduced.
Further, various techniques have been proposed as a technique of further reducing power consumption in the standby mode (see Japanese Unexamined Patent Application Publication Nos. 2002-58238, 2002-58238 and 2000-270544, and Japanese Unexamined Utility Model Application Publication No. H03-113986, for example).
Japanese Unexamined Patent Application Publication No. 2002-58238 discloses a technique of, in the standby mode, stopping the switching of the switching element when the outputted voltage is higher than an upper limit voltage, and starting the switching of the switching element when the outputted voltage is lower than a lower limit voltage. According to this technique, it is possible to increase a bursting cycle by increasing output ripples, and thus, power consumption in the standby mode can be further reduced.
However, according to the technique disclosed in Japanese Unexamined Patent Application Publication No. 2002-58238, during the switching pause period in the standby mode, control power is kept supplied to circuits and elements for controlling driving of the switching element. Accordingly, a power loss occurs in these circuits and elements even during the switching pause period in the standby mode.
By contrast, Japanese Unexamined Patent Application Publication No. 2004-88959 discloses a technique of, in an isolated switch-mode power supply device, providing a switch circuit along a line for supplying control power to the circuits and the elements, and of stopping supplying control power to the circuits and the elements during the switching pause period in the standby mode. According to this technique, it is possible to prevent a power loss from occurring in the circuits and the elements during the switching pause period in the standby mode.
Further, Japanese Unexamined Utility Model Application Publication No. H03-113986 discloses a technique of providing switching means for turning on and off the supply of the control power to the circuits and the elements, and of causing the switching means to turn off the supply of the control power when the outputted voltage exceeds a predetermined value that has been preliminarily determined. According to this technique, it is possible to prevent a power loss from occurring in the circuits and the elements when the outputted voltage exceeds the predetermined value. Here, as the outputted voltage increases during the oscillation period in the standby mode, switching to the switching pause period when the outputted voltage exceeds the predetermined value that has been preliminarily determined can prevent a power loss from occurring in the circuits and the elements during the switching pause period in the standby mode.
Moreover, Japanese Unexamined Patent Application Publication No. 2000-270544 discloses a technique of, in an isolated switch-mode power supply device, providing control power to the circuits and the elements by a startup circuit in which a starting resistance and the switch circuit are connected in series. When the outputted voltage is higher than the upper limit voltage in the standby mode, the isolated switch-mode power supply device stops the switching of the switching element and turns the switch circuit to the OFF state to stop the startup circuit. By contrast, when the outputted voltage is lower than the lower limit voltage in the standby mode, the isolated switch-mode power supply device turns the switch circuit to the ON state and operates the startup circuit, thereby starting the switching of the switching element. According to such an isolated switch-mode power supply device, it is possible to prevent a power loss from occurring due to the starting resistance during the switching pause period in the standby mode.